Blender Path Deform Bone Contraint Deforms Mesh
There is another method for this kind of purpose, which is better than this one, given here by Batfinger. That said your problem is just that your vertices are influenced by other bones, if I select one of the 4 back face vertices, here is what I see (by the way, Bone is probably your root bone, you haven't disabled its Deform option):

So simply click on the X button to remove these vertex groups, or use the Weight Paint Subtract brush.